Letter to the Editor: Do Complication Rates Differ by Gender After Metal-on-metal Hip Resurfacing Arthroplasty? A Systematic Review
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- Amstutz, H. & Le Duff, M. Clin Orthop Relat Res (2015) 473: 3981. doi:10.1007/s11999-015-4561-x
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To the editor,
We read the study by Haughom and colleagues with great interest. We would like to acknowledge the efforts made by the authors to produce a thorough investigation and a high-quality review on the topic.
However, the authors conclude that “moving forward studies should report outcomes by gender (particularly complications).” This statement suggests that gender itself is responsible for differences in the outcome of hip resurfacing arthroplasty. Although an association between female gender and higher failure rates or complications is undeniable as shown in the review by Haughom and colleagues, causation, on the other hand, is certainly not demonstrated. When it comes to determination of risk factors for a procedure, a multivariate model is impractical if the wrong variables are being studied. For hip resurfacing, using gender as one of the variables creates collinearity, and usually precludes the study of variables that are correlated with gender but have a much stronger rationale for affecting the outcome, such as component size [1, 5] or an etiology of developmental dysplasia [1, 3] (which increases the chances of cup malposition, a variable that can only be studied using a three-dimensional (3-D) analysis and calculating the contact patch to rim distance, which has the strongest correlation with elevated wear [4, 6]).
Recent publications [4, 6] have highlighted the importance of proper acetabular component positioning, both in the frontal and sagittal planes for hip resurfacing arthroplasty to be successful, and to avoid edge-loading of the components, leading to abnormal wear patterns of the device [4, 6]. Large databases such as registries certainly present the advantage of high statistical power but completely lack the ability to record the variables that truly affect the outcome of hip resurfacing, such as a 3-D assessment of acetabular component positioning. For patients with small components, the safe zone of implantation is reduced. This certainly applies to many women, but not all of them. Additionally, not all hip resurfacing devices have the same coverage of the head by the socket  and such differences in the small sizes can greatly affect the outcome of hip resurfacing arthroplasty.
This is particularly important in our overly litigated society because every time a publication is made that suggests female gender is a risk factor, it reinforces the use of simplistic treatment algorithms and perfectly suitable female patients get denied the opportunity to be treated with hip resurfacing arthroplasty. Gender is a variable that usually gets included in studies more by tradition and availability for every patient, in every database, than because of its actual value to answer the research question. Women with hip resurfacing arthroplasty do not fail more often than men because they are women, but because of confounding variables that should be the object of study to refine inclusion criteria for the procedure.